Jesus Colon, who identifies as black and Puerto Rican, was faced with a pivotal and life-changing moment in the most ordinary of circumstances in the 1950s. Riding the subway around midnight, he spots a white woman with her hands literally full of children and luggage in need of help. And he hesitates to offer help.
Colon considers her reaction. Perhaps she will scream. Perhaps she will think this black man was trying to get "too familiar." He assesses his own risk and decides to walk by without helping. It is at this point when he exits the subway that "the cold air slaps [his] warm face."
This is an example of personification (air is given the human characteristic of slapping) that is used to show several things. He acts coldly to a woman in need, and the cold air is a reminder of that. It is also close to a common idiom: We say that something "slapped me in the face" when we find it particularly shocking or eye-opening.
Colon reflects that racism turns people into animals. It takes people who would otherwise show a fellow human in need a courtesy and makes them walk right by, seeming not to even notice.
This cold air that slaps him in the face serves to wake Colon up to his own failings. And he resolves that should he ever find himself in the same situation, he will help "regardless of how the offer is going to be received." After all, the little things often are the big things.